The present invention is generally related to the use of ultrasonic energy for the ablation and dissolution of blood clots in vessels and is more particularly directed to the intravascular generation of ultrasonic energy for removal of intravascular blockages by ablation thereof.
Ultrasonic energy has been considered for removal of intravascular blockages due to both atherosclerotic plaque and intravascular blood clots. It has been shown that ultrasonic energy is useful in fragmenting plaque and thrombosis, either as a result of mechanical action thereon or cavitation thereof, in which high energy ultrasound applied to liquids generates vapor-filled microbubbles, or "cavities," with the concomitant rapid expansion and collapse of the cavities, accompanied by local intense hydraulic shock leading to fragmentation or dissolution of the thrombosis.
Heretofore developed equipment for the ultrasonic elimination of plaque and the like has included apparatus for the generation of the ultrasonic energy exterior to the body and the transmission of the ultrasonic energy into the body through a transmission wire to the stenosis site. Although good ultrasonic plaque ablation has been shown using an intravascular device consisting of a 2 mm titanium ball which is vibrationally excited by a 0.015 inch (0.38 mm) diameter titanium wire, these devices do not allow deployment in desired regions due to the stiffness of the transmission wire.
Unfortunately, as the wire diameter is reduced to lower the stiffness thereof, it is more difficult to deliver sufficient energy to the ball for surgical procedures.
As set forth in co-pending application Ser. No. 07/625,919 filed Dec. 10, 1990, a miniature ultrasonic ablation tool mounted at the tip of a flexible catheter and deployed in small diameter arteries is useful for removal of intra arterial atherosclerotic plaque and/or blood clots. This reference is incorporated herewith, including all drawings and specification, by this specific reference thereto.
Naturally, an ultrasonic transducer sized for arterial insertion must be approximately 1/1000 the conventional size. This represents a tremendous reduction in available mechanical output energy and forces an emphasis on careful management of output energy and on understanding energy transfer during the ablation process.
In U.S. Ser. No. 07/625,919, a device is described which allowed the maximum possible transfer of acoustic energy from piezoelectric crystal to the target medium even for devices with diameters of 2 mm and length 8 mm. Acoustic outputs which have been achieved by this device cause ablation and/or dissolution effects. However, the rates are relatively slow.
The improved miniature ultrasonic ablation/dissolution apparatus of the present invention is specifically useful for clot dissolution, as distinguished from plaque ablation. It has been found that with specific structure, as hereinafter discussed, enhanced clot dissolution occurs, without damage to artery walls or significant disturbance to plaque. Hence, the present invention is specifically directed to removal of clot blockages in arteries.